Extrusion feedstock compositions comprising cellulose fiber, for example, wood fiber, and a resin, for example, a thermoplastic resin, for example, polyvinyl chloride resin, are well known. Examples of such compositions are described in U.S. Pat. Nos. 6,103,791 and 6,248,813 to Zehner and U.S. Pat. No. 6,210,792 to Seethamraju et al, which are hereby incorporated by reference. These compositions can be extruded to provide shaped articles, for example, decorative molding for finish carpentry. Compositions based on polyvinyl chloride resins (PVC) are preferred for such applications because of their superior weather-resistant properties over other types of resin compositions.
In general extrusion of material reinforced thermoplastic resin compositions, for example a cellulose/polyvinyl chloride resin composition requires heating an extrudable mixture to a temperature at which it can undergo plastic flow, and then forcing it through a plate containing one or more openings (herein, a “die”) which impart a profile shape to the emerging extrudate. As it emerges from the orifice opening, the extrudate is cooled until it becomes rigid, retaining the profile shape. The shaped extrudate is used either in its extruded form, for example, as a decorative molding, or it is cut into convenient length pieces and packaged for use in follow-on processing, for example, as feedstock for an injection or blow molding process.
In general an extrusion feedstock composition, also referred to herein for convenience as “an extrudable composition”, is often prepared by combining cellulose fiber and a resin, for example, a thermoplastic resin, for example, polyvinyl chloride (PVC) resin, and other constituents (also called additives) which effect the properties of the composition and the extrudate, for example, a lubricant. Other examples of constituents which may be included in an extrudable composition, for example, one containing a PVC resin, are those described in the Handbook of Plastic Materials and Technology, Ed. I. Rubin, Wiley-Interscience, John Wiley & Sons, Inc. New York, 1990 and Plastics Additives and Modifiers Handbook, Ed. J. Edenbaum, Van Nostrand Reinhold, New York 1992, Chapter 3, each of which is incorporated herein by reference. Examples of additional constituents (additives) include fillers (for example, calcium carbonate), processing aids (for example acrylic copolymers), and stabilizers for imparting heat stability to the composition during the extrusion process (for example, tin based stabilizer such as TM281® from Rohm & Haas).
Once combined, the constituents of the composition are “fused,” that is, subjected to heating and blending until there is formed a plastic composition which is substantially homogeneous. The term “fusion” is applied to this process because in heating and blending the boundaries of the individual granules of the constituents become indistinguishable, thus, “fused”. The various additives and the amounts of the additives used influence the properties of the composition and its performance during and after extrusion. Examples of properties effected include the processability of the material during extrusion or in follow-on processes in which the extruded material is employed and the physical and mechanical properties of an extrudate made from the composition. Examples of physical properties are the dimensional stability and surface gloss of the extrudate product prepared from the extrudable composition. Examples of mechanical properties include the flexural and tensile strength of an extrudate prepared from the extrudable composition. Examples of extrudability properties of the extrudable composition include the extrusion rate of the composition for a given extruder torque setting or value of head pressure
Dimensional stability and surface appearance (surface gloss) are characteristics that may affect the commercial value of an extruded product and hence of the processes and the components used to make the extruded product. Dimensional stability is assessed by observing the amount by which an extruded shape expands during solidification after it emerges from the die of an extrusion apparatus. The amount of expansion is sometimes referred to as die swell. Surface gloss is measured in accordance with known standard measurements, for example, ASTM standards D-2523-95 and D2457-97
The tensile and flexural strength of extruded articles are important to the commercial value of an extruded product also. These mechanical properties can be effected by the type and amount of lubricant included in the extrudable composition preparatory to producing the extruded article. These mechanical properties can be measured in accordance with known standard measurements, for example ASTM standards D 638 and D 6109 for measurement of the tensile and flexural strength of an extruded article, respectively
For a given extrudable composition, it is generally possible to increase the rate at which material is extruded. However, such extrusion rate increases have practical limitations. For example, it is possible to obtain extrusion rate increases by operating the extruder at a higher temperature and/or at a higher head pressure. However, at some point of increasing extruder temperature the extrudable composition will generally begin to scorch, and the surface and structural qualities of the extrudate will begin to degrade. Furthermore, as the head pressure or extruder torque is increased, a point is generally reached at which the dimensional stability and/or surface condition of the extruded material is unacceptable. These process limitations have created practical limits on the rate at which prior compositions could be extruded.
Another mechanism that is at least theoretically available to achieve higher extrusion rates is to increase the amount and/or type of lubricant incorporated into the extrudable composition. However, it is also possible that increasing the amount of the lubricant or changing the type of lubricant can have deleterious effects on the extruded product, for example, compromising one or more of the inherent physical and/or mechanical properties of the product (extrudate) and/or on other aspects of the extrusion process.
Reduction in the tensile strength of an extrudate is an example of one inherent mechanical property of an extrudate which may be compromised by increasing the amounts of certain lubricants or other additives. In particular, increasing amounts of certain lubricants known as external lubricants in reinforced resin extrudates, for example, cellulose reinforced PVC extrudates, can reduce the adhesion of the PVC resin to the reinforcing material, negatively impacting the strength of the extrudate and its surface appearance. Also, increased amounts of external lubricant may negatively impact not only the strength of the extrudate but may also increase the fusion time, which increases the overall processing time.
Applicants have thus come to recognize the need for cellulose reinforced extrudable compositions, particularly extrudable PVC-based compositions, having one or more of those desirable extrusion characteristics associated with high levels of lubricant while reducing or substantially eliminating one or more or the adverse effects that would otherwise be associated with a high level of lubricant. The present methods and extrudable compositions meet this and other needs.